I. Field of the Invention
This invention relates to heating systems by which to heat the plastic matrix of a travelling web of fiber reinforced composite tape for application by a tape dispensing apparatus. More particularly, the present invention relates to a heating control system in which heating and temperature control of the plastic matrix of the travelling web is reliably accomplished irrespective of the velocity or changes in velocity of the travelling web.
II. Description of the Prior Art
By way of background, computerized tape laying machines have been utilized to apply laminated strips of composite tape material to a lay-up tool to form a part such as an aircraft wing, for example. One such composite tape material includes graphite fibers embedded in a thermosetting plastic matrix. To apply the tape, a computerized tape laying machine may include a tape dispensing head which is movable under computer control relative to the lay-up tool to lay courses of tape to build up multiple plys of tape, forming the desired part. An example of one such system is described in U S. Pat. No. 4,719,397 assigned to the assignee hereof, the disclosure of which is incorporated herein by reference.
Conventionally, tape dispensing apparatus of the type described in U.S. Pat. No. 4,719,397 have been used to apply thermosetting composite tape to the lay-up tool. Thermosetting composite tape is fairly moldable and usually tacky at or around room temperature. Although many thermosetting composite tapes have some natural adhesive properties in the matrix, bismaleimide (BMI) composite tape has little tack at ambient temperature and, thus, will resist adherence to previous plys of such tape or to a tool surface. Additionally, in its natural state, composite tape material is relatively stiff and virtually inflexible in sidewise directions. i.e., in the plane of the tape.
Certain prior art machines have employed heaters for heating composite tape, from one side of the tape. For example, infrared or quartz bulb heaters have been proposed for heating thermoplastic composite tape, i.e. tape which may be heated to such point where plys may be melted together. Heated shoes and hot air have been employed to heat both thermoplastic and thermosetting composite tape. Additionally, at least one prior art device, shown in U.S. Pat. No. 4,133,711 at FIG. 22 shows that thermosetting tape which is separated from its carrier backing paper for purposes of cutting the tape alone, may be reattached after cutting, by passing the backing paper and cut composite tape through a clamp apparatus which employes a heating coil.
The various movements of the state-of-art machine systems result in widely fluctuating velocities of the tape as it is applied to the surface. For example, the tape may be moving at a velocity between 0 and 20 inches per second, and with possibly dramatic changes in velocity. And, for example, if heater bulbs are energized to emit radiant energy at a level sufficient to heat the plastic matrix when the tape is moving relatively slowly, such as at 1 inch per second, the matrix will not be properly heated when the tape is moving relatively rapidly. Moreover, unless the energy level for the heater is raised accordingly, the tape will not be properly heated as it accelerates. Similarly, if heater bulbs are energized to emit radiant energy at the higher level necessary to heat the plastic matrix during periods of high speed travel of the tape, the tape may be burned or otherwise damaged as the tape velocity is reduced. Thus, heating of the tape must be carefully regulated so that proper heating is achieved without burning or otherwise damaging the tape.
In the case of prior art machines using a heated compaction shoe or roller, very little surface is in contact with the back of the tape, and the front of the tape is laid to an unheated structure, such that insufficient heating of the tape occurs.
We have identified that it is desirable to heat some composite tapes, whether they be unbacked, or comprise a tape assembly where, for instance, the tape is carried on a thin backing paper from the tape supply reel to the compaction shoe.
Tape heating may be desirable for many reasons, two of which are (1) certain types of thermosetting composite tapes, for example, bismaleimide (BMI), have very little tack and it is thus desirable to enhance the tack when creating laminated structures; and (2) in the processing of composite tape structures, lay-up tools may have a variety of contours, and, since the relatively stiff tape does not easily conform to contours into which it is "steered" (i.e., the tape is directed sidewise, in the plane of the tape), and the tape is more steerable at elevated temperatures.
We have determined that composite tape (either alone, or in combination with a backing as a tape structure), is advantageously heated by a heating system which applies heat from both sides of the tape, and preferably, where the heating system comprises apparatus with a defined length along the tape path, located close to the tape laydown zone.